There are applications in which an electrical load is switched by a switch in the form of an NMOS transistor, i.e., an N-channel MOS transistor, with said switch being located on the side of the high potential, i.e., on the side of the high potential of the load. With an NMOS disposed on the high potential side, the source thereof is usually disposed on the supply voltage source on the high potential side. In order to be able to switch the NMOS transistor to the conducting state, a control voltage must be supplied to the gate thereof which must be higher than the voltage present at the source thereof by at least the switching-on threshold value leading to conduction of this transistor, i.e., the gate of this NMOS transistor must be fed with a control voltage that is higher than the supply voltage on the high potential side. To this end, a voltage increasing circuit is required, for example in the form of a bootstrap circuit, an inductive dc-dc voltage converter or a charge pumping circuit. For inexpensive integrated circuits, a charge pumping circuit is preferred most since it does not require external components. When the integrated circuit has to make do with low supply voltages, care has to be taken that minimum internal voltage drops are obtained.
Conventional charge pumping circuits comprise a T-type circuit having two series-connected diodes in the series branch and a pumping capacitor in the shunt branch located therebetween. One of the diodes, on the side remote from the pumping capacitor, is connected to a supply voltage source and permits the charging voltage to pass to the capacitor without permitting discharge of the latter via the supply voltage source. The second diode serves for decoupling the charging or pumping capacitor from a load connected to the end of the second diode remote from the pumping capacitor. With the conventional charge pumping circuit it is thus not possible to avoid a voltage drop corresponding to a double diode voltage drop.
It would be possible now to use instead of diodes switches such as DMOS transistors. However, such switches in turn would have to be driven by means of an additional charge pumping circuit. Switching of such switching transistors for effecting rectification, which in the case of the conventional charge pumping circuit is effected by diodes, results in an increase in EMR (electromagnetic interference radiation) in the range of the basic switching frequency.
Such problems are overcome by an electrical switch device known from JP 5-831 04A, comprising a switch means having a switching control terminal; a control circuit which is connected between a first voltage source terminal and a second voltage source terminal and having a control signal input adapted to have a binary switching control signal applied thereto and having a switching signal output acting on the switching control terminal of the switch means and which contains two control circuit branches connected in parallel to each other, each control circuit branch having a series connection including a first switch, a second switch serving as switchable load impedance for the first switch, and a circuit node located between the first switch and the second switch, with the switching signal output being connected to one of the two circuit nodes, the second switches each having a load impedance switching control terminal and being cross-coupled in that the load impedance switching control terminals thereof are connected to the circuit node of the respective other control circuit branch, the first switches being switchable to the conducting state and to the blocking state in opposite manner by means of the switching control signal such that, when one of the two first switches is switched to the conducting state, the respective other first switch is switched to the blocking state and vice versa, and the second switches, due to their cross-coupling to each other and due to their series connection with the first switches, being switchable on the one hand in opposition to each other to the conducting state and to the blocking state such that, when one of the two second switches is switched to the conducting state, the respective other one of the second switches is switched to the blocking state, and vice versa, and being switchable on the other hand in opposition to the first switches to the blocking state and to the conducting state such that in each control circuit branch, when the first switch thereof is switched to the conducting state, the second switch thereof is switched to the blocking state, and vice versa.
This switch means of the known switching device causes virtually no voltage drop in the conducting state since a conducting MOS transistor, which preferably constitutes the switch means, has only a very low impedance. Due to the control of the switch means by a control circuit of the type mentioned above, no higher voltage is required for switching the switch means to the conducting state than the voltage switched by the switch means.
When replacing passive circuit components, such as diodes, by active circuit components, such as transistor switches, the result is in general an additional power consumption. This is not so in the case of the afore-mentioned switching device whose control circuit is composed such that it effects switching of the switch means in power-free manner. For, in each of the two control circuit branches there is always one switch switched to the non-conducting state.
Preferably both the switch means and the switches of the control circuit are constituted by MOS transistors. It may happen then that undesirably high voltages, which impair the MOS transistors, occur at the circuit nodes, i.e., also at the gate of the MOS transistor constituting the switch means.